Hybrid Technology Helps Fuel Transmission Transformation

Transmission manufacturers such as ZF Friedrichshafen AG are adapting their transmissions for use in plug-in hybrid vehicles by replacing the torque converter with an electric motor.

For example, ZF has developed its own plug-in version of its 8HP eight-speed automatic transmission.

The electric motor powers the transmission when the internal combustion engine is not running.

The top short-term performance of the electric motor can reach 80 kilowatts — enough to allow a two-ton vehicle to run nonstop through the complete European test cycle under electric power.

Depending on the electrical energy storage capacity, it can achieve a nominal carbon dioxide reduction of up to 70 percent.

For now, Schaeffler Group’s Dierk Reitz thinks the integration of the electric motor into the bell housing of the automatic transmission is the best choice. At the Association of German Engineers symposium in late June, he presented a new hybrid module.

One peculiarity of the device: Not all the torque created by the internal combustion engine is guided into the transmission via the electric motor.

The torque is split and is in part passed directly to the transmission’s input shaft, allowing up to 590 pounds-feet of torque.

“We are observing a trend to increasingly high-performance hybrid vehicles,” Reitz said. Both Schaeffler and ZF also want to use some of the output to actively dampen vibrations.

While the new Schaeffler and ZF solutions await their first series applications, an electrically reworked variation of the Mercedes-Benz seven-speed automatic transmission S class is already in regular production.

Almost all the providers harbor doubts about whether the strategy pursued so far is the best one. Reitz calculates that the fuel consumption advantage of numerous gears will steadily shrink in hybrid vehicles. Some measurements show that there is less than a 1 percent difference between a five-speed transmission and a nine-speed transmission.

“That encouraged us to give more thought to the simplification of the transmission,” Reitz told Auto Released.

Schaeffler has under development an electrified three-speed transmission for vehicles with range extenders.